Activision Blizzard has confirmed that the fifth playable character class in Diablo 4 will be the Necromancer. It will be the first Diablo game to feature an open world and offer character customization options. The game will be available to play on PC and consoles (current-gen and previous) sometime in 2023. https://www.notebookcheck.net/New-Diablo-4-gameplay-trailer-reveals-the-fifth-playable-character-class-open-world-map-and-a-2023-launch-date.628373.0.html
Definitely not the first to support local multiplayer. Unless you don't count D2 having Lan as an option without needing a battle.net account and if you also don't count the D3 couch co-op.
The first to be open world? I know diablo 2 was a mix between open world and labyrinthic, but it has never been corridor like or hand hold (like immortal), unless you mean that the transition between places is seamless and without a single way in and out, which would now be truly different from other diablo games
And customizable characters? Diablo has always been customizable, skill trees and specs have always been a stapple, its has that rpg characteristic after all, unless you meant that the character you choose has facial and body configurations
I feel this whole written thing is scripted by someone on blizzard and people in this webpage have never played a Diablo game maybe beyond immor(t)al, barely read about the other games and try to project their ideas of the series in a comparative way, a blind leading the blind
Click bait topic, not worth the reading i gave it